In the interests of conserving energy, windows with a low emissivity (low-E) coating have grown in popularity over the years. Such windows are now common in new commercial construction. These windows offer the benefits of conserving energy and money by reducing the need for air conditioning and/or heating. However, such windows have a severe impact on radio frequency (RF) signals. The coatings that are applied to window glass to transform it to a low-E window are comprised mainly of metals. Although the coatings are thin and transparent, their metallic content is effective at conducting electricity. This makes the coatings efficient reflectors of broad bands of radio frequency signals. Signals transmitted through the windows can be attenuated at levels of 30 decibels (dB) or more. Furthermore, commercial construction tends to use other materials that further block RF signals. Materials such as concrete, brick, mortar, steel, aluminum, roofing tar, gypsum wall board, and some types of wood all offer varying degrees of RF absorption. The result is that many newer commercial buildings severely impede RF signals from getting into or out of the buildings.
Nonetheless, RF devices have become an important part of modem life, especially with the emergence of smartphones, which have a variety of RF devices built-in. Such devices may include cellular transceivers, wireless local area network (“wi-fi”) transceivers, Global Positioning System (GPS) receivers, Bluetooth transceivers and, in some cases, other RF receivers (e.g., FM/AM radio, UHF, etc.). As the popularity of such devices has grown, the importance of being able to use RF-based features within the confines of modem commercial buildings has grown.
Currently there are several commercially available techniques to facilitate the use of smartphones or similar devices within affected buildings. All of these techniques pose challenges, both for the installer as well as the user, depending on the technique. They include the following:
First, an external antenna can be connected to most cell phones in order to improve antenna gain. However, there still must be sufficient signal within the building and the cell phone must remain connected via a cable. This solution can be inconvenient and has performance limitations in some circumstances.
Second, a femtocell can be used. A femtocell is a device that resembles a wireless router and can be purchased from mobile service providers. This device connects to an existing broadband internet connection in a home or office, and radiates cell phone signals to a maximum range of about 40 feet. The maximum coverage per device is approximately 5000 square feet. Depending on the mobile provider, these devices can support between four and 16 simultaneous calls and require data bandwidths of greater than 1.5 Mb/s. Typically, prepaid phones are not supported with this solution. Also, these devices must be located near a window to provide a GPS signal. However, while a femtocell will support cellular signals, low-E coated windows still block GPS signals. This latter problem can be addressed by an external GPS antenna connected to the femtocell. Although connections initiated via the femtocell automatically connect to cell towers when the phone moves outdoors during a call, it does not work the other way—calls are dropped if the connection is established outdoors via a cell tower and the phone is then moved indoors where a femtocell is located. Another disadvantage of using femtocells in office buildings is that they are not universal for all mobile providers. A different femtocell must be purchased and set up for each provider.
The third solution is the deployment of a cellular repeater or booster that consists of an external antenna, a bi-directional amplifier, and internal antennas for re-transmission. These can be installed by individual carriers, whose equipment will only work for that carrier, or by independent installers who can provide multi-carrier and multi-band equipment. Second generation (2G) and third generation (3G) technologies all operate in the 850 MHz (cellular) and 1900 MHz (PCS) bands. All major carriers (e.g., Verizon, AT&T, Sprint and T-Mobile) operate in this dual-band region. Fourth generation (4G) systems operate at 700 MHz, 1700/2100 MHz and require different antennas than the 2G/3G dual bands. Most currently deployed repeaters were designed for 3G technology and do not operate at 4G frequencies. 5-band amplifiers, which operate over both 3G and 4G bands, have recently become available, but are likely not compatible with currently deployed antennas and will probably be incompatible with the next generation of cellular hardware. Cellular repeaters also do not boost Wi-Fi, WiMax and other signals in the 5 GHz band. In general, the use of repeaters will constantly require upgrades as technology advances.
There are additional limitations to the use of repeaters. Most applications require a custom installation. There must be room for antennas to be placed on the roof and cables must be run from them to interior locations. There is also a tradeoff between using antennas which provide a high gain but are unidirectional, and thus only boost signals from a single cell tower, and omni-directional antennas that provide less gain. The strongest amplifiers have enough gain to provide indoor coverage of up to about 100,000 square feet. This number is typically much lower, though, due to interior building materials that provide obstacles to the signal and unusual building geometries. In addition, concrete and steel floors significantly attenuate the signals, resulting in the need for separate amplifiers on every floor. Moreover, the cost of outfitting a commercial building can exceed $10K per floor.
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